A urea SCR system has been developed as an exhaust gas purification system for an engine (especially, diesel engine). The urea SCR system has a selective reduction type NOx catalyst and an oxidation catalyst in an exhaust pipe in this series. A urea water adding valve is provided upstream of the NOx catalyst to add urea water into the exhaust pipe. The urea water is hydrolyzed by exhaust heat to generate ammonia (NH3). The NOx catalyst adsorbs the ammonia. The NOx is purified by reductive reaction between ammonia and NOx on the NOx catalyst.
As shown in FIG. 2, a limit value of ammonia adsorption quantity varies according to NOx catalyst temperature. The limit value of ammonia adsorption quantity becomes small as the NOx catalyst temperature increases. Thus, when the NOx catalyst temperature increases, a limit value of ammonia adsorption quantity becomes small so that excess ammonia flows out from the NOx catalyst. The oxidation catalyst oxidizes the excess ammonia. The oxidation catalyst degrades the ammonia into nitrogen (N2) and water (H2O).
However, when ammonia quantity flowing out from the NOx catalyst, which is referred to as ammonia slip quantity, excess a capacity of the oxidation catalyst, the oxidation reaction in the oxidation catalyst is saturated. Then, the ammonia flows out from the oxidation catalyst and is released into the atmosphere. Besides, in the urea SCR system, a NOx sensor detecting NOx quantity (NOx concentration) is disposed between the NOx catalyst and the oxidation catalyst. If the ammonia flows out from the NOx catalyst, the NOx sensor outputs an erroneous signal. That is, NOx sensor detects not only NOx but also ammonia. Thus, when the ammonia flows out from the NOx catalyst, the NOx sensor outputs a detection signal indicative of NOx quantity which is larger than an actual NOx quantity in exhaust gas.
In a system shown in JP-2007-162487A (US-2007-0160508A1), when the NOx catalyst temperature exceeds a specified temperature in which the ammonia starts to desorb from the NOx catalyst, an addition of the urea water by the urea water adding valve is stopped Thereby, it is restricted that ammonia flows out from the NOx catalyst.
However, since the temperature in which the ammonia starts to desorb from the NOx catalyst varies according to the ammonia adsorption quantity, a computation of the ammonia adsorption quantity is necessary. In order to compute the ammonia adsorption quantity, a urea water adding quantity, NOx quantity flowing into the NOx catalyst and NOx quantity flowing out from the NOx catalyst should be identified. These quantities include error. Thus, it can not correctly detect whether ammonia flows out from the NOx catalyst.